Field evaluation of the response of Aedes albopictus (Stegomyia albopicta) to three oviposition attractants and different ovitrap placements using black and clear autocidal ovitraps in a rural area of Same,Timor‐Leste

Known oviposition attractants or stimulants were compared, singly and in combination, using inexpensive autocidal ovitraps designed to trap emerging adults, in a rural area of Timor‐Leste during the dry season. In this area, the dengue vector Aedes albopictus (Stegomyia albopicta) Skuse (Diptera: Culicidae) was abundant, but Aedes aegypti (Stegomyia aegypti) L. was not detected. The attractants were: (a) a compound found in Aedes eggs (dodecanoic acid); (b) components of nitrogen, phosphorous and potassium‐based (NPK) fertilizer, and (c) infusions of discarded cigarette butts. A solution of ammonium phosphate and potassium nitrate was significantly more attractive to gravid Ae. albopictus than water only. Dodecanoic acid and cigarette butt infusions were not significantly more attractive than the control; however, they attracted various other Diptera and many non‐culicid larvae developed in ovitraps in which these substances were used; thus, the presence of eggs or larvae of other species may have deterred Aedes oviposition. Significantly more Aedes eggs were found in ovitraps under vegetation than in ovitraps placed inside houses or against external walls. Clear‐sided ovitraps in which black mesh was placed over a black ring floating on the water surface collected significantly fewer eggs than black ovitraps with identically placed mesh and rings.     

Prospective field study of transovarial dengue‐virus transmission by two different forms of Aedes aegypti in an urban area of Bangkok,Thailand

A prospective field study was conducted to determine transovarial dengue‐virus transmission in two forms of Aedes aegypti mosquitoes in an urban district of Bangkok, Thailand. Immature Aedes mosquitoes were collected monthly for one year and reared continuously until adulthood in the laboratory. Mosquitoes assayed for dengue virus were processed in pools and their dengue virus infection status was determined by one‐step RT‐PCR and nested‐PCR methods. Of a total 15,457 newly emerged adult Ae. aegypti, 98.2% were dark and 1.8% of the pale form. The results showed that the minimum infection rate (MIR) by transovarial transmission (TOT) of dengue virus during the one‐year study ranged between 0 to 24.4/1,000 mosquitoes. Dengue virus TOT increased gradually during the hot summer months, reaching a peak in April‐June, while dengue cases peaked in September, a rainy month near the end of the rainy season. Therefore, mosquito infections due to TOT were prevalent four months before a high incidence of human infections. TOT dengue virus infections occurred in both forms of Ae. aegypti. All four dengue serotypes were detected, with DEN‐4 predominant, followed by DEN‐3, DEN‐1, and DEN‐2, respectively.     

Seasonal population dynamics and the genetic structure of the mosquito vector Aedes aegypti in São Paulo,Brazil

Population genetic studies of insect vectors can generate knowledge to improve epidemiological studies focused on the decrease of pathogen transmission. In this study, we used nine SNPs across the Aedes aegypti genome to characterize seasonal population variations of this important dengue vector. Mosquito samples were obtained by ovitraps placed over Botucatu SP from 2005 to 2010. Our data show that, regardless of the large variation in mosquito abundance (deduced from the number of eggs obtained from ovitraps), the effective population size remained stable over the years. These results suggest that Ae. aegypti is able to maintain a sufficiently large active breeding population during the dry season to keep genetic frequencies stable. These results open new perspectives on mosquito survey and control methods.     

Natural vertical transmission of dengue viruses in Aedes aegypti in selected sites in Cebu City,Philippines

We attempted to determine the vertical transmission of dengue virus (DENV) in Aedes aegypti in selected sites in Cebu City, Philippines. Mosquito sub‐adults were collected monthly from households and the field during the wet‐dry‐wet season from November, 2011 to July, 2012 and were laboratory‐reared to adults. Viral RNA extracts in mosquitoes were assayed by hemi‐nested RT‐PCR. Results showed that 62 (36.26%; n=679) out of 171 mosquito pools (n=2,871) were DENV+. The minimum infection rate (MIR) of DENV ranged from 0 in wet months to 48.22/1,000 mosquitoes in April, 2012 (mid‐dry). DENVs were detected in larvae, pupae, and male and female adults, with DENV‐4, DENV‐3, and DENV‐1, in that rank of prevalence. DENV‐1 co‐infected with either DENV‐3 or ?4 or with both in April, 2012; DENV‐3 and ?4 were present in both seasons. More DENV+ mosquitoes were collected from households than in field premises (p<0.001) and in the dry than in the wet season (p<0.05), with significant interaction (p<0.05) between sites and premises but no interaction between sites and seasons (p>0.05). By Generalized Linear Mixed models, the type of premises nested in sites and monthly total rainfall were significant predictors of monthly dengue cases (p<0.05) and not MIR, season, temperature, and relative humidity. Surveillance of DENV prevalence in Ae. aegypti and detecting their natural foci in the dry season provide an early warning signal of dengue outbreak.     

Consequences of the introduction of the planarian Girardia anceps (Tricladida: Dugesiidae) in artificial containers with larvae of the mosquitoes Aedes aegypti and Culex pipiens (Diptera: Culicidae) from Argentina

Aedes aegypti and Culex pipiens are container-dwelling mosquito species that are vectors of important diseases to man, such as dengue and lymphatic filariasis, respectively. Predators of these pests are an interesting alternative to be incorporated to biological control measures. We tested the consequences of introducing individuals of Girardia anceps, a native freshwater flatworm species, within artificial water containers where larvae of these mosquitoes thrive. Our goals were to ascertain if mosquito species, density of larvae (high or low), type of water container (tires or ovitraps), and presence or absence of planarians affected mosquito survivorship (measured as number of individuals reaching the pupa stage) in manipulated artificial containers. Furthermore, we monitored ovitraps in the field along several months in order to explore the long-term effect of the presence of planarian on the colonization of these containers by feral mosquitoes under natural conditions. We found that the presence of planarians reduced the number of mosquitoes reaching pupation and that such reduction depends on the initial density of larvae. Reduction of populations of A. aegypti was high along the breeding season of this mosquito, being the effect less evident in C. pipiens. G. anceps could be an agent of control against container-breeding mosquitoes if its release in small water containers is complemented with other suitable management strategies.     

Spatio-temporal distribution of Aedes mosquitoes in Car Nicobar Island: Implication in the transmission of arboviruses

Arboviral infections, viz. dengue and chikungunya are prevalent in the Andaman & Nicobar Islands. During post-tsunami developmental activities, large plastic tanks were provided to the native, Nicobarese tribal households of Car Nicobar Island, to store water for domestic use. These tanks form an ideal breeding source for mosquitoes, especially the vectors of dengue/chikungunya viruses, and few cases of IgM ELISA positives for these infections were identified from this island. In view of this scenario, a survey was carried out to determine the prevalence of these mosquito vectors. Ten randomly selected clusters (neighborhoods with 50 houses each) were surveyed. Each household was inspected for the water holding receptacles. This was the first attempt to determine the prevalence and distribution of the vectors of dengue/chikungunya virus in this Island, against the backdrop of various post tsunami rehabilitation and developmental activities. The stegomyia indices with respect to houses and containers were high during the winter and post-monsoon periods (House Index and Container Index were 69.40 and 46.41 in winter, while 54.40 and 39.49 in post-monsoon). Large plastic tanks (500–1000 l capacity) recorded the highest Breteau Index during all the four seasons. This habitat was observed to support four mosquito species, of which 52% constituted Aedes albopictus. The pupae/person index ranged from 0 to 0.2946. A community-based control approach with multiple stakeholders is envisaged to prevent the vector breeding. This approach would be feasible and effective, with active participation of the tribal chieftain along with village headmen.     

An Explosive Epidemic of DENV-3 in Cairns,Australia

From November 2008-May 2009 Cairns Queensland Australia was struck by an explosive epidemic of DENV-3 that exceeded the capacity of highly skilled dengue control team to control it. We describe the environmental, virological and entomological factors associated with this outbreak to better understand the circumstances leading to its occurrence. Patient interviews, serological results and viral sequencing strongly suggest that the imported index case was infected in Kalimantan, Indonesia. A delay in notification of 27 days from importation of the index case until Queensland Health was notified of dengue transmission allowed the virus to amplify and spread unchecked through November 2008. Unseasonably warm weather, with daily mean temperatures exceeding 30°C, occurred in late November and would have shortened the extrinsic incubation period of the virus and enhanced transmission. Analysis of case movements early in the outbreak indicated that the total incubation period was as low as 9–11 days. This was supported by laboratory vector competence studies that found transmission by Aedes aegypti occurred within 5 days post exposure at 28°C. Effective vector competence rates calculated from these transmission studies indicate that early transmission contributed to the explosive dengue transmission observed in this outbreak. Collections from BG sentinel traps and double sticky ovitraps showed that large populations of the vector Ae. aegypti occurred in the transmission areas from November – December 2008. Finally, the seasonal movement of people around the Christmas holiday season enhanced the spread of DENV-3. These results suggest that a strain of DENV-3 with an unusually rapid transmission cycle was able to outpace vector control efforts, especially those reliant upon delayed action control such as lethal ovitraps.     

Development and laboratory evaluation of chemically‐based baited ovitrap for the monitoring of Aedes aegypti

Aedes (Stegomyia) aegypti is considered to be the most important dengue vector worldwide. Studies were conducted to design and evaluate a chemically‐based baited ovitrap for monitoring Ae. aegypti under laboratory conditions. Several known chemical attractants and three types of ovitraps (ovitraps A, B, and C) were evaluated throughout the oviposition bioassays. Oviposition responses of gravid female Ae. aegypti were evaluated to n‐heneicosane, 3‐methylindole (skatole), 4‐methylphenol (p‐cresol), and phenol. Female Ae. aegypti were attracted to all the evaluated compounds. Among them, n‐heneicosane at a concentration of 10 ppm (mg/l), skatole from 50 to 1000 ppm, p‐cresol at 100 ppm, and phenol at 50 ppm showed a significant positive oviposition response. A blend of the four chemical attractants increased the oviposition response; 67% of the eggs were deposited in the treatment compared to the control. Female Ae. aegypti were signi?cantly more attracted to ovitrap A loaded with the four‐component synthetic blend compared to the standard ovitrap in the oviposition bioassays. The compound used in ovitrap A retained its attractant property for up to three days. The chemically‐based baited ovitrap may be considered as an option to be integrated during the monitoring of dengue virus vectors in México.     

Vertical oviposition activity of mosquitoes in the Atlantic Forest of Brazil with emphasis on the sylvan vector,Haemagogus leucocelaenus (Diptera: Culicidae)

This study aimed to assess the vertical patterns of oviposition and temporal changes in the distribution of mosquito species in an area of the Atlantic Forest in Rio de Janeiro State, Brazil, and in particular, the behavior and oviposition of potential yellow fever virus vectors. Mosquito samples were collected from the Ecological Reserve Guapiaçu (REGUA, Brazil), which includes a somewhat disturbed forest, with a large diversity of plants and animals. In all, 5,458 specimens (ten species from seven genera) were collected. Haemagogus leucocelaenus was the most frequently captured species, representing 73% of the specimens collected. Species richness and diversity were the highest in the samples collected from the ground‐level ovitraps and decreased with height. Species composition also differed significantly among heights. The largest species differences were detected between ovitraps set at the ground level and those set at 7 m and 9 m; Hg. leucocelaenus, Limatus durhamii, and Limatus paraensis contributed most to these differences. Sampling month and climatic variables had significant effects on species richness and diversity. Species diversity and richness decreased with height, suggesting that the conditions for mosquito breeding are more favorable closer to the ground. Species composition also showed vertical differences.     

Trapping biases of Culex torrentium and Culex pipiens revealed by comparison of captures in CDC traps,ovitraps, and gravid traps

We evaluate three trapping methods for their effectiveness at capturing Culex pipiens and Culex torrentium, both enzootic vectors of bird‐associated viruses in Europe. The comparisons, performed in two regions in Sweden, were among CDC traps baited with carbon dioxide, gravid traps, and ovitraps baited with hay infusion. The proportions of the two Culex species in a catch differed between trap types, with CDC traps catching a lower proportion of Cx. torrentium than both gravid traps and ovitraps. Between gravid traps and ovitraps, there was no difference in the proportions of the two species. The results indicate that Cx. torrentium may go undetected or underestimated compared to Cx. pipiens when using carbon dioxide baited CDC traps. The new insight of trap bias presented here adds an important dimension to consider when investigating these vectors of bird‐associated viruses in the field.     

Meteorologically Driven Simulations of Dengue Epidemics in San Juan,PR

Meteorological factors influence dengue virus ecology by modulating vector mosquito population dynamics, viral replication, and transmission. Dynamic modeling techniques can be used to examine how interactions among meteorological variables, vectors and the dengue virus influence transmission. We developed a dengue fever simulation model by coupling a dynamic simulation model for Aedes aegypti, the primary mosquito vector for dengue, with a basic epidemiological Susceptible-Exposed-Infectious-Recovered (SEIR) model. Employing a Monte Carlo approach, we simulated dengue transmission during the period of 2010–2013 in San Juan, PR, where dengue fever is endemic. The results of 9600 simulations using varied model parameters were evaluated by statistical comparison (r²) with surveillance data of dengue cases reported to the Centers for Disease Control and Prevention. To identify the most influential parameters associated with dengue virus transmission for each period the top 1% of best-fit model simulations were retained and compared. Using the top simulations, dengue cases were simulated well for 2010 (r² = 0.90, p = 0.03), 2011 (r² = 0.83, p = 0.05), and 2012 (r² = 0.94, p = 0.01); however, simulations were weaker for 2013 (r² = 0.25, p = 0.25) and the entire four-year period (r² = 0.44, p = 0.002). Analysis of parameter values from retained simulations revealed that rain dependent container habitats were more prevalent in best-fitting simulations during the wetter 2010 and 2011 years, while human managed (i.e. manually filled) container habitats were more prevalent in best-fitting simulations during the drier 2012 and 2013 years. The simulations further indicate that rainfall strongly modulates the timing of dengue (e.g., epidemics occurred earlier during rainy years) while temperature modulates the annual number of dengue fever cases. Our results suggest that meteorological factors have a time-variable influence on dengue transmission relative to other important environmental and human factors.     

Natural vertical transmission of dengue viruses by Aedes aegypti in Bolivia

The natural transmission of dengue virus from an infected female mosquito to its progeny, namely the vertical transmission, was researched in wild caught Aedes aegypti during an important outbreak in the town of Santa Cruz de la Sierra, Bolivia. Mosquitoes were collected at the preimaginal stages (eggs, larvae and pupae) then reared up to adult stage for viral detection using molecular methods. Dengue virus serotypes 1 and 3 were found to be co-circulating with significant higher prevalence in male than in female mosquitoes. Of the 97 pools of Ae. aegypti (n = 635 male and 748 female specimens) screened, 14 pools, collected in February-May in 2007, were found positive for dengue virus infection: five DEN-1 and nine DEN-3. The average true infection rate (TIR) and minimum infection rate (MIR) were respectively 1.08% and 1.01%. These observations suggest that vertical transmission of dengue virus may be detected in vectors at the peak of an outbreak as well as several months before an epidemic occurs in human population.     

Arbovirus detection in synanthropic mosquitoes from the Brazilian Amazon and in mosquito saliva using Flinders Technology Associates cards

Although arbovirus transmission and identifying target vectors may provide a baseline for planning disease control strategies, there are many gaps in knowledge regarding these mosquitoes and viral species in urban, rural, or sylvatic habitats in the Brazilian Amazon. Our goal was to screen for dengue, chikungunya, and Zika viruses in synanthropic mosquitoes and with Flinders Technology Associates (FTA) cards using insect saliva. Mosquitoes were caught using ovitraps and aspirators in the city of Porto Velho, Rondônia, Brazil. Honey-baited FTA cards were placed in mosquito cages for 15 days; whole mosquitoes and FTA cards were analysed for viral RNA using RT-qPCR assays. One pool of Aedes aegypti females was found to be infected with the Zika virus and one male mosquito was infected with dengue-4, suggesting natural vertical/venereal transmission. Our study also reported evidence of vertical/venereal transmission of ZIKV in Culex quinquefasciatus males for the first time in the Brazilian Amazon, and the feasibility of using FTA cards to detect arboviruses in the saliva of field-collected mosquitoes. Vertical/venereal transmission of viruses by atypical mosquito species reinforces the need for combined viral and entomological screening in arbovirus surveillance programs.     

A decade of colonization: the spread of the Asian tiger mosquito in Pennsylvania and implications for disease risk

In recent decades, the Asian tiger mosquito expanded its geographic range throughout the northeastern United States, including Pennsylvania. The establishment of Aedes albopictus in novel areas raises significant public health concerns, since this species is a highly competent vector of several arboviruses, including chikungunya, West Nile, and dengue. In this study, we used geographic information systems (GIS) to examine a decade of colonization by Ae. albopictus throughout Pennsylvania between 2001 and 2010. We examined the spatial and temporal distribution of Ae. albopictus using spatial statistical analysis and examined the risk of dengue virus transmission using a model that captures the probability of transmission. Our findings show that since 2001, the Ae. albopictus population in Pennsylvania has increased, becoming established and expanding in range throughout much of the state. Since 2010, imported cases of dengue fever have been recorded in Pennsylvania. Imported cases of dengue, in combination with summer temperatures conducive for virus transmission, raise the risk of local disease transmission.     

Satellite observation to assess dengue risk due to Aedes aegypti and Aedes albopictus in a subtropical city of Argentina

Earth observation environmental features measured through remote sensing and models of vector mosquitoes species Aedes aegypti and Ae. albopictus provide an advancement with regards to dengue risk in urban environments of subtropical areas of Argentina. The authors aim to estimate the effect of landscape coverage and spectral indices (Normalized Difference Vegetation Index [NDVI], Normalized Difference Water Index [NDWI] and Normalized Difference Built-up Index [NDBI]) on the larvae abundance of Ae. aegypti and Ae. albopictus in Eldorado, Misiones, Argentina using remote satellite sensors. Larvae of these species were collected monthly (June 2016 to April 2018), in four environments: tire repair shops, cemeteries, dwellings and an urban natural park. The proportion of landscape coverage (water, urban areas, bare soil, low vegetation and high vegetation) was determined from the supervised classification of Sentinel-2 images and spectral indices, calculated. The authors developed spatial models of both vector species by generalized linear mixed models. The model's results showed that Ae. aegypti larvae abundance was better modelled by NDVI minimum values, NDBI maximum values and the interaction between them. For Ae. albopictus proportion of bare soil, low vegetation and the interaction between both variables explained better the abundance.     

Wolbachia infection alters the relative abundance of resident bacteria in adult Aedes aegypti mosquitoes,but not larvae

Insect–symbiont interactions are known to play key roles in host functions and fitness. The common insect endosymbiont Wolbachia can reduce the ability of several human pathogens, including arboviruses and the malaria parasite, to replicate in insect hosts. Wolbachia does not naturally infect Aedes aegypti, the primary vector of dengue virus, but transinfected Ae. aegypti have antidengue virus properties and are currently being trialled as a dengue biocontrol strategy. Here, we assess the impact of Wolbachia infection of Ae. aegypti on the microbiome of wild mosquito populations (adults and larvae) collected from release sites in Cairns, Australia, by profiling the 16S rRNA gene using next‐generation sequencing. Our data indicate that Wolbachia reduces the relative abundance of a large proportion of bacterial taxa in Ae. aegypti adults, that is in accordance with the known pathogen‐blocking effects of Wolbachia on a variety of bacteria and viruses. In adults, several of the most abundant bacterial genera were found to undergo significant shifts in relative abundance. However, the genera showing the greatest changes in relative abundance in Wolbachia‐infected adults represented a low proportion of the total microbiome. In addition, there was little effect of Wolbachia infection on the relative abundance of bacterial taxa in larvae, or on species diversity (accounting for species richness and evenness together) detected in adults or larvae. These results offer insight into the effects of Wolbachia on the Ae. aegypti microbiome in a native setting, an important consideration for field releases of Wolbachia into the population.     

Selective and frequency dependent predation of aquatic mosquito predator Diplonychus indicus Venkatesan & Rao (Hemiptera: Belostomatidae) on immature stages of three mosquito species

Frequency dependent mosquito larval size (II and IV instars) and species selection by the water bug Diplonychus indicus against three mosquito species Culex quinquefasciatus, Aedes aegypti and Anopheles stephensi was studied in the laboratory. The different frequencies used for each species selection were 20:30:50, 30:50:20, 50:20:30, 25:35:40, 35:40:25 and 40:25:35 of fourth instars of the respective three prey species. All nymphal water bugs (I–V instars) selected IV instar mosquito larvae and the mean proportion of late (larger) larvae eaten by the predator instars was significantly higher than the mean proportion of early (smaller) larvae eaten (F= 2.28; P < 0.001). In all six ratios used to determine the frequency dependent mosquito species selection, all the stages of the water bug selected Ae. aegypti over the other two species (F= 452.43; P < 0.001). The mean number of mosquito larvae eaten increased as its density increased based on various ratios of larvae offered. The study indicated that the predatory efficiency of D. indicus was high when Ae. aegypti was offered as prey, suggesting the utility of this mosquito predator in the control of dengue vectors.